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96 Cards in this Set

  • Front
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  • 3rd side (hint)
In soft tissue, the greatest backscatter factor is associated with:
a) 30 kVp x-rays
b) 4 MV linac x-rays
c) Co-60
d) 2mm Al HVL x-rays
e) 1mm Cu HVL x-rays
e) 1mm Cu HVL x-rays
Scatter is associated w/COMPTON INTERACTIONS.
@2mm Al HVL= mainly Photo Electric Interactions.
@about 1m Cu HVL and above =mainly Compton.
As photon E increases, more energy is transferred to the e- and less to the scattered photon. The max BSF is~0.7mm Cu HVL.
The TMR depends on:
a) Energy, SAD, Depth & Field Size
b) Energy, SAD & Field Size
c) SAD, Depth & Field Size
d) Energy, Depth & Field Size
e) SSD only
d) Energy, Depth & Field Size
TMR, like TAR, is independent of SAD
TAR is:
a) Equal to the BSF @ dmax
b) Independent of SAD
c) Used in calc of timer settings in rotational therapy
d) all the above
e) none of the above
d) all the above
The timer or MU setting for rotation uses the ave TAR, averaged over all depths for the area of rotation
A single PA spine field is tx'd @130cm SSD. Compared to tx @ 80cm SSD, the exit dose will be:
a) Greater
b) Smaller
c) the same
a) Greater
As SSD increases, PDD (and thus Exit Dose) increase - per Mayneord's f-factor
Which is true:
a) TAR Increases as SSD Increases
b) BSF Increases as beam Energy Increases above 1 MV
c) PDD Increases with Increasing SSD
d) TMR can NOT be measured for Co-60
c) PDD Increases with Increasing SSD
a) TAR is INDEPENDENT of SSD and SAD.
b) BSF increases with increasing energy up to~1mm Cu HVL, then Decreases as Energy increases.
d) Historically, TARs measured for Co-60 & TMRs measured for Higher Energy beams. *But, TMRs can be measured & used for calc-ing timer settings AT ANY MEGAVOLTAGE ENERGY*
TMR:
Dependent/Independent of SAD
TMR, like TAR, is INDEPENDENT of SAD (and SSD).
TAR:
Dependent/Independent of SAD
TAR, like TMR, is INDEPENDENT of SAD (and SSD).
TAR:
Dependent/Independent of SSD
TAR is INDEPENDENT of SSD (and SAD).
TMR, formula:
a) (dose rate at dmax) / (dose rate at depth) at SSD
b) (dose rate at depth) / (dose rate at dmax) at SSD
c) (dose rate at dmax) / (dose rate at depth) at SAD
d) (dose rate at depth) / (dose rate at dmax) at SAD
e) (dose rate at dmax) / (dose rate in air) at SAD
d) (dose rate at depth) / (dose rate at dmax) at SAD
BSF, formula:
a) (dose rate at dmax) / (dose rate at depth) at SSD
b) (dose rate at depth) / (dose rate at dmax) at SSD
c) (dose rate at dmax) / (dose rate at depth) at SAD
d) (dose rate at depth) / (dose rate at dmax) at SAD
e) (dose rate at dmax) / (dose rate in air) at SAD
e) (dose rate at dmax) / (dose rate in air) at SAD
PDD/100, formula:
a) (dose rate at dmax) / (dose rate at depth) at SSD
b) (dose rate at depth) / (dose rate at dmax) at SSD
c) (dose rate at dmax) / (dose rate at depth) at SAD
d) (dose rate at depth) / (dose rate at dmax) at SAD
e) (dose rate at dmax) / (dose rate in air) at SAD
b) (dose rate at depth) / (dose rate at dmax) at SSD
TMR:
a) stands for Tumor-maximum ratio
b) is the ratio of dose at dmax to dose at depth
c) Increases as SSD Increases
d) can NOT be measured on a Co-60 unit
e) NONE of the above
e) NONE of the above.
TMR=Tissue-maximum ratio.
It is the ratio of two dose rates measured in a phantom, at the SAME distance from the source (one w/a chosen thickness of overlaying phantom, the other w/only the rhicness rquired to attain dmax.
It is Independent of SSD.
It can be measured on any megavoltage x-ray or gamma ray unit.
TMR:
a) Tissue-maximum ratio.
b) the ratio of two dose rates measured in a phantom, at the SAME distance from the source
c) Independent of SSD.
d) can be measured on any megavoltage x-ray or gamma ray unit.
e) all of the above
e) all of the above
TMR=Tissue-maximum ratio.
It is the ratio of two dose rates measured in a phantom, at the SAME distance from the source (one w/a chosen thickness of overlaying phantom, the other w/only the rhicness rquired to attain dmax.
It is Independent of SSD.
It can be measured on any megavoltage x-ray or gamma ray unit.
BSF:
a) is the TAR @ dmax
b) Increases as energy Increases over 1MV
c) is the PDD @ dmax
d) is the ratio of dose in air to dose in tissue
e) All the above
a) is the TAR @ dmax
a) TAR is the dose rate at depth / dose rate in air at the same point.
The TAR @ dmax is called the Back Scatter Factor.
BSF DECREASES as Energy INCREASES above 1 MV
All of the following are INDEPENDENT of SSD - EXCEPT:
a) TMR
b) TAR
c) PDD
d) BSF
c) PDD
c) PDD:
PDD INCREASES with INCREASING SSD - since it contains an inverse square (IVS) component as well as attenuation.
TMR, TAR and BSF measure attenuation only, and are INDEPENDENT of SSD
BSF: Increases/Decreases as Energy Increases above 1 MV
BSF DECREASES as Energy INCREASES above 1 MV
TAR (definition)
TAR is:
the dose rate at depth / dose rate in air at the same point
BSF (definition)
TAR @ dmax
At Energies > 1MV:
BSF DECREASES
PDD -vs- SSD:
a) Proportional
b) Inversely Proportional
c) Independent
PDD and SSD are:
PROPORTIONAL
PDD -vs- SSD, example:
PDD INCREASES with INCREASING SSD
TMR, TAR and BSF measure (?) only
TMR, TAR and BSF measure ATTENUATION Only
TMR, TAR and BSF measure attenuation only, and are INDEPENDENT of SSD
TMR -vs- SSD
a) Proportional
b) Inversely Proportional
c) Independent
TMR is INDEPENDENT of SSD
TMR, TAR and BSF measure attenuation only, and are INDEPENDENT of SSD
TAR -vs- SSD
a) Proportional
b) Inversely Proportional
c) Independent
TAR is INDEPENDENT of SSD
TMR, TAR and BSF measure attenuation only, and are INDEPENDENT of SSD
BSF -vs- SSD
a) Proportional
b) Inversely Proportional
c) Independent
BSF is INDEPENDENT of SSD
TMR, TAR and BSF measure attenuation only, and are INDEPENDENT of SSD
10x10 field, Superficial (2.5mm Al HVL); BSF= (?)
a) 1.0
b) 1.02
c) 1.035
d) 1.15
e) 1.26
e) Superficial (2.5mm Al HVL); BSF= 1.26
The BSF is a function of:
Beam Quality AND Field Size.
BSF can NOT have a value of 1.0 (Because that would imply that there is NO difference between the dose rare in air & that at dmax in tissue).
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV.
10x10 field, Co-60; BSF= (?)
a) 1.0
b) 1.02
c) 1.035
d) 1.15
e) 1.26
e) Co-60; BSF= 1.035
The BSF is a function of:
Beam Quality AND Field Size.
BSF can NOT have a value of 1.0 (Because that would imply that there is NO difference between the dose rare in air & that at dmax in tissue).
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV.
10x10 field, 10MV x-rays; BSF= (?)
a) 1.0
b) 1.02
c) 1.035
d) 1.15
e) 1.26
b) 10MV x-rays; BSF= 1.02
The BSF is a function of:
Beam Quality AND Field Size.
BSF can NOT have a value of 1.0 (Because that would imply that there is NO difference between the dose rare in air & that at dmax in tissue).
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV.
The BSF is a function of:
(?) AND (?).
The BSF is a function of:
Beam Quality AND Field Size.
The BSF is a function of:
Beam Quality AND Field Size.
BSF can NOT have a value of 1.0 (Because that would imply that there is NO difference between the dose rare in air & that at dmax in tissue).
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV.
BSF:
Can/Can NOT be=1.00
BSF can NOT have a value of 1.0 (Because that would imply that there is NO difference between the dose rare in air & that at dmax in tissue).
The BSF is a function of:
Beam Quality AND Field Size.
BSF can NOT have a value of 1.0 (Because that would imply that there is NO difference between the dose rare in air & that at dmax in tissue).
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV.
BSF Increases to a value of ~ (?) for a large field @ ~1mm Cu HVL
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, (then falls to a negligable value > 10 MV)
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > (? energy)
(BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL), then falls to a negligable value > 10 MV
BSF Increases to a value of ~1.5 for a large field @ ~1mm Cu HVL, then falls to a negligable value > 10 MV
Which is FALSE about TMR:
a) it is = TAR/BSF
b) it is ~ related to the %DD by Inverse Square factor
c) it is the ratio of the dose at depth / the dose at dmax, both measured at the isocenter
d) it is Dependent on SSD
e) it Increases with Increasing Field Size
d) it is Dependent on SSD - is FALSE.
TMR is INDEPENDENT of SSD (since the dose at depth and the dose at dmax are measured at the same distance from the source)
True/False, TMR is = TAR/BSF
TRUE:
TMR is = TAR/BSF
True/False, TMR is approximately related to the %DD by Inverse Square factor
TRUE:
TMR is approximately related to the %DD by Inverse Square factor
True/False, TMR is the ratio of the dose at depth / the dose at dmax, both measured at the isocenter
TRUE:
TMR is the ratio of the dose at depth / the dose at dmax, both measured at the isocenter
True/False, TMR Increases with Increasing Field Size
TRUE:
TMR Increases with Increasing Field Size
TMR is: Dependent/Independent of SSD? (BONUS: Why?)
TMR is INDEPENDENT of SSD (since the dose at depth and the dose at dmax are measured at the same distance from the source)
TMR is INDEPENDENT of SSD (since the dose at depth and the dose at dmax are measured at the same distance from the source)
TAR @ dmax can be calculated from the BSF by:
a) multiplying by the SAR
b) dividing BSF by the collimator output factor
c) applying Inverse Square correction to the BSF
d) no need to calculate; they are the SAME @ dmax
d) no need to calculate; they are the SAME @ dmax
TAR = BSF in what situation?
TAR = BSF @ dmax
For megavoltage photons, TMR has replaced TAR because:
a) TMR is INDEPENDENT for Field Size
b) TAR is difficult to measure at high energies
c) TMR is preferable to TAR for rotation calculations
d) TMR is independent of depth of mazimum dose
b) TAR is difficult to measure at high energies
As the beam energy ↑, the depth of dmax also ↑. A large size build-up cap will start to act as a "mini" phantom.
As Beam Energy ↑, Depth of dmax:
a) Increases
b) Decreases
c) Unchanged
a) As the beam energy ↑, the depth of dmax also ↑.
Given a square & rectangle of the same area, which would have the greater % DD for Co-60:
a) square
b) rectangle
c) they have the SAME depth dose
a) square
Scatter from the corners of the retangle has FARTHER to travel tha from the periphery of the square & will contribute less to the depth dose.
The TAR for a 10x10cm field @ 100cm SAD for a 4 MB photons is 0.8 at a depth of 7cm. What change would you expect in the TAR by extending the SSD from 93 cm to 193 (200cm from the source), for the same field size @ SAD:
a) 5% Increase in TAR
b) 10% Increase in TAR
c) 15% Increase in TAR
d) NO Change in TAR
d) NO Change in TAR
TAR is INDEPENDENT of SSD
As Photon Energy ↑, the TMR @ dmax:
a) Increases
b) Decreases
c) Remains Unchanged
c) Remains Unchanged
TMR @ dmax is 1.0 by definition for ANY photon energy
As Photon Energy ↑, the % transmission thru a 1cm Lucite blocking tray:
a) Increases
b) Decreases
c) Remains Unchanged
a) Increases
Which of the follwing is CORRECT:
a) TAR = TMR x BSF
b) TAR = TMR / BSF
c) TMR = BSF / TAR
d) TMR = TAR x BSF
e) BSF = TAR x TMR
a) TAR = TMR x BSF
TMR
= (dose rate @ depth) / (dose rate @ dmax)
= (dose rate @ depth) / (dose rate in air x BSF)
= TAR/BSF; therfore TAR = TMR x BSF
% DD in photon beams:
1. Increases w/Increasing SSD
2. Increases w/Increasing Field Size
3. Increases w/Increasing beam Energy
4. Decreases exponentially (not including inverse square effect & scattering) beyond dmax:
a) 1 only
b) 1, 2, 3
c) 2, 4
d) 4 only
e) All are correct
e) All are correct
% DD Increases w/Increasing SSD, according to Mayneord's f-factor, because the Inverse Square Factor becomes relatively less important at greater SSD
True/False:
% DD in photon beams Increases w/Increasing SSD
True:
% DD in photon beams Increases w/Increasing SSD
% DD Increases w/Increasing SSD, according to Mayneord's f-factor, because the Inverse Square Factor becomes relatively less important at greater SSD
True/False:
% DD in photon beams Increases w/Increasing Field Size
True:
% DD in photon beams Increases w/Increasing Field Size
% DD Increases w/Increasing SSD, according to Mayneord's f-factor, because the Inverse Square Factor becomes relatively less important at greater SSD
True/False:
% DD in photon beams Increases w/Increasing beam Energy
True:
% DD in photon beams Increases w/Increasing beam Energy
% DD Increases w/Increasing SSD, according to Mayneord's f-factor, because the Inverse Square Factor becomes relatively less important at greater SSD
True/False:
% DD in photon beams Decreases exponentially (not including inverse square effect & scattering) beyond dmax
True:
% DD in photon beams Decreases exponentially (not including inverse square effect & scattering) beyond dmax:
% DD Increases w/Increasing SSD, according to Mayneord's f-factor, because the Inverse Square Factor becomes relatively less important at greater SSD
Relative Output Factor of Co-60 for a 5x5 Field Size is (?)
Relative Output Factor of Co-60 for a 5x5 Field Size is: 0.96
The relative Output Factor for Co-60 range from ~ 0.96 (5x5 field) to 1.07 (30x30 field)
Relative Output Factor of Co-60 for a 30x30 Field Size is (?)
Relative Output Factor of Co-60 for a 30x30 Field Size is: 1.07
The relative Output Factor for Co-60 range from ~ 0.96 (5x5 field) to 1.07 (30x30 field)
At 80cm SAD, the dose rate for a 10x10cm field using Co-60 is 100cGy/min in air. The dose rate in air for a 30x30cm field would be (cGy/min):
a) 101
b) 103
c) 107
d) 115
e) 125
c) 107
The relative Output Factor for Co-60 range from ~ 0.96 (5x5 field) to 1.07 (30x30 field)
True/False:
The Relative Output Factor range for a Linac is > the relative Output Factor range for Co-60
True:
The Relative Output Factor range for a Linac is > the relative Output Factor range for Co-60
The Relative Output Factor range for a Linac is > the relative Output Factor range for Co-60 - DUE TO SCATTER INTO THE ION CHAMBER FROM THE COLLIMATORS
A 6MV linac is calibrated @ the iso @ depth dmax (D). To calculate the MUs, MU = (dose@depth)/x, where x = (?)
a) D x BSF x TAR
b) D x TAR
c) D x TMR
d) D x BSF x TMR
e) none of the above
c) D x TMR
The general time-on formula is: time = (dose @depth)/dose rate @ depth).
True/False -
Dose Rate at depth can be found by either using:
Dose Rate in Air (SAD x TAR) or
Dose Rate in Tissue at SAD (dmax x TMR)
True.

(NOTE: BSF x TMR = TAR)
Dose Rate at depth can be found by either using:
Dose Rate in Air (SAD x TAR) or
Dose Rate in Tissue at SAD (dmax x TMR)
BSF x TMR = ?
BSF x TMR = TAR
To calculate Dose Rate at depth, Dose Rate in Air = ? x ?
Dose Rate in Air = SAD x TAR
To calculate Dose Rate at depth,
Dose Rate in Tissue at SAD = ? x ?
Dose Rate in Tissue at SAD = dmax x TMR
The dose @ dmax, for an SSD calculation, (formula):
D= ? / ?
The dose @ dmax, for an SSD calculation:
D = Prescribed dose / %DD
10x10 Field Size @ 80cm SSD. Depth=7cm. %DD=65%. Dose Rate @ dmax=100cgy/min. Presc dose=150cGy@d7cm, dose@dmax= (?)
D = Prescribed dose / %DD
D = 150cGy / 65%
D = 150 / 0.65
D = 231cGy
The dose @ dmax, for an SSD calculation:
D = Prescribed dose / %DD
A patient was previously tx'd (single AP sclav field, SSD setup). Before treating an adjacent spine field, what data is needed to figure prior cord dose:
1. Given dose to s'clav field
2. Field Size
3. Depth to cord
4. %DD tables
5. Timer error
6. Dose rate in tissue

a) 1, 2, 3, 4
b) 1, 2, 3, 4, 5
c) 2, 3, 4
d) 1, 2, 4, 5, 6
e) 3, 4, 6
a) 1, 2, 3, 4
Exit to cord = given dose to s'clav x %DD. (%DD is a function of field size & cord depth)
True/False:
The Equivalent Square of a rectangular field -
Has the Same Area as the Rectangle
False
(The Eq Sq of a rectangle has a SMALLER area than the rectangle)
*The Eq Sq of a rectangular field is that square field which has the same scatter contribution on the axis - and therfore the same %DD and TAR. (One sq cm near the axis contributes MORE scatter than the same area at the corner of the field - therfore the Eq Sq of a rectangle has a SMALLER area than the rectangle)
True/False:
The Equivalent Square of a rectangular field -
Is approx twice the area divided by the perimeter
False
An approx rule for the side of the Eq Sq ~ (4xA)/P, where A=Area & P=Perimeter
True/False:
The Equivalent Square of a rectangular field -
Has the same %DD on the axis as the Rectangular field
True
True/False:
The Equivalent Square of a rectangular field -
Has the same backscatter factor as the Rectangular field
True
BSF = TAR @ dmax
Given a circle and a rectangle of the same area, which would have the greater %DD?
a) the circle
b) the rectangle
c) they have the SAME depth dose
a) the circle
Scatter from the corners of the rectangle has farther to travel than from the periphery of the circle - and therefore will contribute LESS to the to the depth dose
The % of dose @ dmax will be:
Greater/Less
for Smaller fields.
Bonus: Why?
The % of dose @ dmax will be:
GREATER for Smaller fields.
(Due to the change of %DD w/Field Size)
The % of dose @ dmax will be:
Greater/Less
for Larger fields.
Bonus: Why?
The % of dose @ dmax will be:
LESS for Larger fields.
(Due to the change of %DD w/Field Size)
Lowest cord dose results from:
Higher/Lower energy AND Smaller/Larger SSD?
HIGHEST Energy and LARGEST SSD
The lowest dose to points between midplane and the surface results from using the HIGHEST Energy and LARGEST SSD (ex: SSD setup would be larger than Isocentric), to maximize %DD.
The LOWEST dose to points between midplane & the surface results from using the (Highest/Lowest) energy and the (Largest/Smallest) SSD.
Bonus: Why?
The LOWEST dose to points between midplane & the surface results from using the HIGHEST energy and the LARGEST SSD.
Bonus: To maximize %DD
Total dose delivered @ d=dmax from a pair of parallel opposed fields, expressed as a % of the total dose at midplane:
a) Decreases as photon energy Increases
b) Decreases as Field Size Increases
c) Increases as patient thickness increases
d) is slightly less for an SSD setup than for an SAD setup
e) All of the Above
f) None of the above
e) All of the Above
Any factor that INCREASES %DD will DECREASE the total dose @dmax, compared w/the Total Dose at midplane. Treating at SSD rather than SAD gives slightly higher %DDs.
Any factor that INCREASES %DD will (INCREASE/DECREASE the total dose @dmax, compared w/the Total Dose at midplane.
Any factor that INCREASES %DD will DECREASE the total dose @dmax, compared w/the Total Dose at midplane.
?? In order to keep the total dose @ dmax<110% of the midplane dose (for Co-60 unit, parallel opposed fields, 80 SSD), AP sep should not be greater than:
a) 8cm
b) 14cm
c) 20cm
d) 24cm
e) 28cm
c) 20cm
Note: the % at dmax will be GREATER for SMALLER fields, and LESS for LARGER fields - due to the change of %DD with field size
Rule of thumb for Co-60 is (?)% extra dose @dmax for (?)cm separation (for SSD setup).
(10%) extra dose @dmax for (20cm) separation (for SSD setup).
Rule of thumb for Co-60 is (10%) extra dose @dmax for (20cm) separation (for SSD setup).
The Mayneord
factor is used for, correction of:
The Mayneord
factor is used for, correction of:
PDDs from one SSD to another
the inverse square law correction component is the main component of
the correction, and is referred to as the Mayneord factor. The second factor,
represented by the ratio of TARs or PSFs, is often ignored, because its effect is
much smaller than that produced by the Mayneord factor, and the Mayneord
factor alone is used for correction of PDDs from one SSD to another
True/False - Using 2 wedged fields, a uniform dose distribution is usually obtained:
Only when the wedges are used at 90degrees to each other
FALSE
Positioning is NOT limited to 90 degrees
True/False - Using 2 wedged fields, a uniform dose distribution is usually obtained:
Only when a 3rd open field is added
FALSE
Uniform dose distributions can be obtained in many cases w/only 2 wedged fields. a 3rd unwedged field is sometimes ued to good effect w/a pair of wedges
True/False - Using 2 wedged fields, a uniform dose distribution is usually obtained:
When the wedge angle is approx 90degrees minus half the hinge angle
TRUE
This is the ideal relation between Hinge Angle & Wedge Angle
True/False - Using 2 wedged fields, a uniform dose distribution is usually obtained:
When the thick ends of the wedges are adjacent to each other
TRUE
For most treatments, the "heels" should be together
The angle between the beam axis in a wedged pair is 60degrees. The appropriate wedge angle would be:
a) 15
b) 30
c) 45
d) 60
d) 60degree wedge
Wedge Angle (WA) = 90degrees - (Hinge angle/2).

90 - (60/2) = 90 - 30 = 60
Wedge Angle (WA), formula is (?)
Wedge Angle (WA) = 90degrees - (Hinge angle/2).
Wedge Angle (definition)
The angle through which the 50% isodose curve is turned (from its position in the open field)
Given:
Output, WTF (wedge factor), TAR, Dose (per beam) - what's the formula to calc MUs?
MU=Dose/(output x WTF x TAR)
ex:
Dose per field=60cGy
Output in air @ iso=0.90cGy/MU
TAR=0.782
WTF=0.58
MU=60/(.9 x .58 x .782)
MU=150/0.59
MU=254
Surface Dose:
(Increases/Decreases) as Photon Energy Increases?
Surface Dose DECREASES as Photon energy INCREASES
Surface Dose DECREASES as Photon energy INCREASES.
The opposite is true for Electrons.
Surface Dose:
(Increases/Decreases) with the addition of a blocking tray?
Surface dose INCREASES with the addition of a blocking tray
Surface dose INCREASES with the addition of a blocking tray
Surface Dose:
(Increases/Decreases) as the SSD Increases for the same field size on the skin?
Surface Dose DECREASES as the SSD INCREASES for the same field size on the skin
Surface Dose DECREASES as the SSD INCREASES for the same field size on the skin
Surface Dose:
(Depends/does NOT Depend) on the obliquity of the patient's skin surface?
Surface Dose DEPENDS on the obliquity of the patient's skin surface
Surface Dose DEPENDS on the obliquity of the patient's skin surface
Surface Dose is primarily due to electrons generated in materials between the (?) & (?)
the Source & the Patient
Surface Dose is primarily due to electrons generated in materials between the (SOURCE) & (PATIENT)
For Co-60 unit:
The skin dose will (Increase/Decrease) when the SSD is Decreased?
The skin dose will (INCREASE) when the SSD is Decreased
As SSD Decreases, scatter from the collimators will INCREASE
For Co-60 unit:
The skin dose will (Increase/Decrease) when the field size is Decreased?
The skin dose will (DECREASE) when the field size is Decreased
Similarly, skin dose will INCREASE as the Field Size INCREASES
For Co-60 unit:
The skin dose will (Increase/Decrease) when bolus is used?
The skin dose will (INCREASE) when bolus is used
Bolus generallly used to decrease skin sparing & bring the skin dose up to 100%
For Co-60 unit:
The skin dose will (Increase/Decrease) when fields are treated at oblique incidence?
The skin dose will (INCREASE) when fields are treated at oblique incidence
Oblq incidence causes secondary electrons to travel in a more parallel (rather than perpendicular) direction to the skin. The buildup depth is reduced and the skin dose INCREASES
Skin dose:
The skin dose is (Higher/Lower) for Larger Field Size?
Skin dose increases w/Larger field size
True/False:
2 AP/PA adjacent photon fields are needed, it's a good idea to have the same collimator settings for both fields because...a larger field size will diverge into a smaller field size & create hot and cold spots
TRUE:
With 2 adjacent AP/PA fields, a larger field size will diverge into a smaller field size & create hot and cold spots
The formula used to caluclate the gap on the skin between adjacent fields, matched @ depth, relies on the fact that:
a) the projection of the edge of the light field shows the 50% decrement line of the radiation field
b) both beams must be treated w/the same energy x-ray beam
c) the angle of divergence of adjacent beams is the same
d) both fields must be treated simultaneously
e) the depth at the junction <10cm
a) the projection of the edge of the light field shows the 50% decrement line of the radiation field